Semiconductor memory device

ABSTRACT

In one embodiment, a semiconductor memory device includes a substrate, and device regions in the substrate to extend in a first direction. The device further includes select gates on the substrate to extend in a second direction, and a contact region provided between the select gates and including contact plugs on the respective device regions. The contact region includes partial regions, in each of which N contact plugs are disposed on N successive device regions to be arranged on a straight line being non-parallel to the first and second directions, where N is an integer of 2 or more. The contact region includes the partial regions of at least two types whose values of N are different. Further, each of the contact plugs has a planar shape of an ellipse, and is arranged so that a major axis of the ellipse is tilted with respect to the first direction.

CROSS REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2011-42515, filed on Feb. 28, 2011, the entire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate to a semiconductor memory device.

BACKGROUND

Recently, semiconductor memory devices such as NAND flash memories have been included in various electronic devices.

Due to requirements for providing such electronic devices with multiple functions, a semiconductor memory device has been required to have large memory capacity. Accordingly, memory cells of the semiconductor memory device have been required to be miniaturized.

A NAND flash memory will be described as an example. In a typical NAND flash memory, a plurality of memory cell transistors are electrically connected in series to form a NAND cell unit. One end of the NAND cell unit is electrically connected to a bit line via a select gate transistor, and the other end of the unit is electrically connected to a source line via another select gate transistor.

Recently, due to miniaturization of a memory cell array, densification and integration in a memory have been advanced. Accordingly, bit line contacts arranged between drain-side select gates have also been required to be reduced in size. However, it is difficult to simply reduce the size of the contacts and the space between the contacts, because they may cause the short of adjacent contacts, decrease in breakdown voltage between the contacts, increase in contact resistance due to the miniaturization, optical interference in lithography due to the shortened distance between the contacts, and the like.

Therefore, attempts to solve these problems have been recently made by using a staggered contact arrangement. An example of the staggered contact arrangement is a two-contact staggered arrangement in which two contacts are the repetition unit of a staggered structure. However, due to the advance of the miniaturization of the memory, the distance between adjacent contacts tends to be shorter, so that a simple two-contact staggered arrangement cannot solve the above problems.

Therefore, it is required to provide such a contact arrangement that can suppress the short of the contacts and the decrease in breakdown voltage between the contacts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view showing a structure of a semiconductor memory device of a first embodiment;

FIG. 2 is a side cross-sectional view taken along a line I-I′ shown in FIG. 1;

FIG. 3 is a side cross-sectional view taken along a line J-J′ shown in FIG. 1;

FIG. 4 is an enlarged plan view showing a region Z shown in FIG. 1;

FIG. 5 is a plan view for explaining a rotation angle of each contact plug;

FIG. 6 is a plan view for explaining a method of determining the rotation angle of the contact plugs;

FIG. 7 is a graph for explaining the method of determining the rotation angle of the contact plugs;

FIGS. 8A to 8C are plan views for explaining a method of changing ellipse dimensions of each contact plug;

FIG. 9 is a plan view showing a structure of a semiconductor memory device of a first comparative example;

FIG. 10 is a plan view showing a structure of a semiconductor memory device of a second comparative example;

FIG. 11 is a plan view showing a structure of a semiconductor memory device of a third comparative example; and

FIG. 12 is a plan view showing a structure of a semiconductor memory device of a second embodiment.

DETAILED DESCRIPTION

Embodiments will now be explained with reference to the accompanying drawings.

An embodiment described herein is a semiconductor memory device including a substrate, and device regions formed in the substrate to extend in a first direction which is parallel to a principal plane of the substrate. The device further includes select gates disposed on the substrate to extend in a second direction which is perpendicular to the first direction, and a contact region provided between the select gates on the substrate and including contact plugs disposed on the respective device regions. Further, the contact region includes partial regions, in each of which N contact plugs are disposed on N successive device regions to be arranged on a straight line being non-parallel to the first and second directions, where N is an integer of 2 or more. Further, the contact region includes the partial regions of at least two types whose values of N are different. Further, each of the contact plugs has a planar shape of an ellipse or rectangle, and is arranged so that a major axis of the ellipse or a long side of the rectangle is tilted with respect to the first direction.

First Embodiment

FIG. 1 is a plan view showing a structure of a semiconductor memory device of a first embodiment. The semiconductor memory device of FIG. 1 is a NAND flash memory.

FIG. 1 shows a memory cell array of the semiconductor memory device of the first embodiment. In the memory cell array of FIG. 1, memory cell transistors 201 and select gate transistors 202 are arranged in a two-dimensional array on a substrate 101. FIG. 1 shows X and Y directions which are parallel to the principal plane of the substrate 101 and are perpendicular to each other. The X direction corresponds to the channel width direction of those transistors, and the Y direction corresponds to the gate length direction of those transistors.

Further, FIG. 1 shows device regions 111 formed in the substrate 101. The device regions 111 are formed in the substrate 101 to extend in the Y direction and to be adjacent to each other in the X direction. The Y direction is an example of a first direction of the disclosure, and the X direction is an example of a second direction of the disclosure. The device regions 111 are also called active areas (AA). As shown in FIG. 1, the memory cell transistors 201 and the select gate transistors 202 are formed on the device regions 111.

Further, FIG. 1 shows isolation insulators 112 which are formed in the substrate 101 and isolate the device regions 111 from each other. In the first embodiment, the isolation insulators 112 are shallow trench isolation (STI) insulators.

In addition, as shown in FIG. 1, NAND strings 211 extending in the Y direction are formed on the individual device regions 111. Each of the NAND strings 211 has memory cell transistors 201 arranged in a line, and two select gate transistors 202 arranged so as to sandwich these memory cell transistors 201. In FIG. 1, the NAND strings 211 are arranged in the X direction to form the memory cell array.

Further, FIG. 1 shows word lines WL₁ to WL_(K) extending in the X direction, and select gates SG_(S) and SG_(D) extending in the X direction, where K denotes an integer of 2 or more. The select gate SG_(S) corresponds to a source-side select gate, and the select gate SG_(D) corresponds to a drain-side select gate. These select gates SG_(S) and SG_(D) and the word lines WL₁ to WL_(K) are commonly connected to gate electrodes of the select gate transistors 202 and the memory cell transistors 201 adjacent to each other in the X direction, respectively. Further, FIG. 1 shows select gates SG_(S)′ and SG_(D)′ extending in the X direction and adjacent to the select gates SG_(S) and SG_(D), respectively.

Further, FIG. 1 shows bit lines BL₁ to BL₃ extending in the Y direction. The bit lines BL₁ to BL₃ are arranged on the device region 111. As shown in FIG. 1, the memory cell transistors 201 are provided at intersections of the word lines WL₁ to WL_(K) and the individual device regions 111. In addition, the select gate transistors 202 are provided at the intersections of the select gates SG_(S) and SG_(D) (and SG_(S)′ and SG_(D)′) and the individual device regions 111.

Further, FIG. 1 shows contact regions R_(S) and R_(D) provided on the substrate 101.

The contact region R_(S) is provided between the source-side select gates SG_(S) and SG_(S)′ on the substrate 101. The contact region R_(S) includes contact plugs (not shown in FIG. 1 for expediency), which are formed on the individual device regions 111 between the source-side select gates SG_(S) and SG_(S)′.

In addition, the contact region R_(D) is provided between the drain-side select gates SG_(D) and SG_(D)′. Similarly to the contact region R_(S), the contact region R_(D) includes contact plugs (not shown in FIG. 1 for expediency), which are formed on the individual device regions 111 between the drain-side select gates SG_(D) and SG_(D)′.

Further, the detail of these contact plugs will be described later.

FIG. 2 is a side cross-sectional view taken along a line I-I′ shown in FIG. 1. FIG. 2 shows the AA (Active Area) cross section of the semiconductor memory device of the first embodiment.

Similarly to FIG. 1, FIG. 2 shows the substrate 101, the device regions 111, and the isolation insulators 112. Further, FIG. 2 shows a buried well region 102 and a well region 103 successively formed in the substrate 101.

In the first embodiment, the substrate 101 is a semiconductor substrate such as a silicon substrate. The buried well region 102 is an N-type well into which N-type impurities are implanted. For example, the N-type impurities are P (phosphorus). The well region 103 is a P-type well into which P-type impurities are implanted. For example, the P-type impurities are B (boron).

Further, FIG. 2 shows isolation trenches T formed in the well region 103. The device regions 111 are defined in the well region 103 by the isolation trenches T to extend in the Y direction. In addition, the isolation insulators 112 are buried into the isolation trenches T and isolate the device regions 111 from each other.

Further, similarly to FIG. 1, FIG. 2 shows the memory cell transistors 201 formed on the device regions 111.

Each of the memory cell transistors 201 includes a gate insulator 121, a floating gate 122, an inter-gate insulator 123, and a control gate 124. The floating gate 122 is formed on a device region 111 via the gate insulator 121. The control gate 124 is formed on the floating gate 122 via the inter-gate insulator 123.

In the first embodiment, while the gate insulator 121 and the floating gate 122 are divided for each memory cell transistor 201, the inter-gate insulator 123 and the control gate 124 are shared among the memory cell transistors 201 adjacent to each other in the X direction. The control gate 124 shown in FIG. 2 corresponds to the word line WL_(K) shown in FIG. 1.

In addition, in the first embodiment, as shown in FIG. 2, the upper surfaces of the isolation insulators 112 are set lower than the upper surfaces of the floating gates 122. As a result, the lower surface of the inter-gate insulator 123 located on the isolation insulators 112 is lower than the lower surface of the inter-gate insulator 123 located on the floating gates 122. Similarly, the lower surface of the control gate 124 located on the isolation insulators 112 is lower than the lower surface of the control gate 124 located on the floating gates 122.

Further, FIG. 2 shows an inter layer dielectric 131 formed on the substrate 101 so as to cover the memory cell transistors 201. The bit lines BL₁ to BL₃ are formed on the inter layer dielectric 131. Further, FIG. 2 shows an inter layer dielectric 132 formed on the inter layer dielectric 131 so as to cover the bit lines BL₁ to BL₃.

FIG. 3 is a side cross-sectional view taken along a line J-J′ shown in FIG. 1. FIG. 3 shows the GC (Gate Conductor) cross section of the semiconductor memory device of the first embodiment.

Similarly to FIG. 2, FIG. 3 shows the substrate 101, the buried well region 102, and the well region 103.

Further, FIG. 3 shows a plurality of memory cell transistors 201 and two select gate transistors 202 formed on the well region 103 (more specifically, on a device region 111).

Each of the select gate transistors 202 includes a first insulating layer 141, a first electrode layer 142, a second insulating layer 143, and a second electrode layer 144. The first electrode layer 142 is formed on the well region 103 via the first insulating layer 141. The second electrode layer 144 is formed on the first electrode layer 142 via the second insulating layer 143. The first electrode layer 142 and the second electrode layer 144 are electrically connected by an opening H provided in the second insulating layer 143.

FIG. 3 shows such memory cell transistors 201 and select gate transistors 202 that form a NAND string 211. These transistors are electrically connected in series by diffusion layers 151 formed in the well region 103. The control gates 124 shown in FIG. 3 are commonly connected among the memory cell transistors 201 adjacent to each other in the X direction so as to be the word lines WL₁ to WL_(K) shown in FIG. 1. The second electrode layers 144 shown in FIG. 3 are commonly connected among the select gate transistors 202 adjacent to each other in the X direction so as to be the select gates SG_(S) and SG_(D) shown in FIG. 1.

Further, similarly to FIG. 2, FIG. 3 shows the inter layer dielectric 131 which covers these transistors, the bit line BL₁ formed on the inter layer dielectric 131, and the inter layer dielectric 132 which covers the bit line BL₁.

(1) Structure of Contact Region R_(D) (Staggered Arrangement)

Hereinafter, the structure of the contact region R_(D) shown in FIG. 1 will be described in detail.

FIG. 4 is an enlarged plan view showing a region Z shown in FIG. 1. FIG. 4 shows the contact region R_(D) provided between the drain-side select gates SG_(D) and SG_(D)′.

FIG. 4 shows contact plugs CW formed on the individual device regions 111. The subscripts added to the symbols CW are for distinguishing these contact plugs CW. These contact plugs CW are bit line contacts. The bottom surfaces of the contact plugs CW are contacted with the device regions 111, and the top surfaces of the contact plugs CW are contacted with the bit lines BL. In this way, in FIG. 4, one contact plug CW is arranged on one device region 111, and one contact plug CW is similarly arranged under one bit line BL.

The contact region R_(D) of FIG. 4 includes first partial regions R₁ and second partial regions R₂.

In each first partial region R₁, three contact plugs CW are formed on three device regions 111 which are successively adjacent to each other in the X direction, so as to be arranged on a straight line which is non-parallel to the X and Y directions. For example, the contact plugs CW₃₁, CW₂₃, and CW₁₅ are formed on three successive device regions 111 so as to be arranged on a straight line L₁. In addition, the contact plugs CW₄₁, CW₃₃, and CW₂₅ are formed on three successive device regions 111 so as to be arranged on a straight line L₂. In this way, in each first partial region R₁, three contact plugs CW are disposed in a three-contact oblique arrangement.

In addition, in each second partial region R₂, two contact plugs CW are formed on two device regions 111 which are successively adjacent to each other in the X direction, so as to be arranged on a straight line which is non-parallel to the X and Y directions. For example, the contact plugs CW₃₂ and CW₂₄ are formed on two successive device regions 111 so as to be arranged on a straight line L₃. In addition, the contact plugs CW₄₂ and CW₃₄ are formed on two successive device regions 111 so as to be arranged on a straight line L₄. In this way, in each second partial region R₂, two contact plugs CW are disposed in a two-contact oblique arrangement.

In the contact region R_(D) of FIG. 4, the first partial regions R₁ and the second partial regions R₂ are alternately provided along the X direction. In this way, the contact region R_(D) of FIG. 4 has a contact arrangement in which three-contact oblique arrangements and two-contact oblique arrangements are alternately repeated.

Consequently, the contact region R_(D) of FIG. 4 has a contact arrangement in which irregular five-contact staggered arrangements are periodically repeated. Therefore, the contact arrangement shown in FIG. 4 will be referred to as an irregular five-contact (three-contact+two-contact) staggered arrangement.

In FIG. 4, the contact plugs CW are arranged on the straight lines L₁ to L₄ which are non-parallel to the X and Y directions. To improve the lithography margin, the straight lines L₁ to L₄ are desirably parallel to each other.

In the first embodiment, N₁ contact plugs CW may be obliquely arranged in each first partial region R₁, where N₁ denotes a constant integer of 2 or more, and N₂ contact plugs CW may be obliquely arranged in each second partial region R₂, where N₂ denotes a constant integer of 2 or more and is different from N₁. In this case, the difference between N₁ and N₂ may be +1 or −1, so that an irregular (N₁+N₂)-contact staggered arrangement can be realized. However, the difference between N₁ and N₂ may be a value other than ±1.

(2) Structure of Contact Plugs CW

Next, the structure of the contact plugs shown in FIG. 4 will be described in detail.

In the first embodiment, as shown in FIG. 4, each contact plug CW in the contact region R_(D) has the planar shape of an ellipse. In addition, each contact plug CW in the contact region R_(D) is disposed so that the major axis of the ellipse is tilted with respect to the Y direction. The tilt of the major axis of the ellipse will be described in detail with reference to FIG. 5.

FIG. 5 is a plan view for explaining a rotation angle of each contact plug CW. The rotation angle means the tilt angle of the major axis of the contact plug CW with respect to the Y direction. Referring to FIG. 5, the rotation angle will be described by taking the contact plug CW₃₁ disposed on the straight line L₁ as an example.

FIG. 5 shows the contact plug CW₃₁ having the planar shape of an ellipse. In FIG. 5, a straight line on the major axis of the ellipse is indicated by Da, and another straight line on the minor axis of the ellipse is indicated by Db. Further, the intersection of the straight lines Da and Db is indicated by G. The point G corresponds to the center point (center axis) of the contact plug CW₃₁. Further, FIG. 5 shows a straight line Y₁ which passes through the point G and extends in the Y direction. In addition, in FIG. 5, the angle of the straight line L₁ with respect to the Y direction is indicated by θ. As described above, the straight line L₁ is non-parallel with the X and Y directions, so that the angle θ satisfies the relation of 0°<θ<90°.

Further, in FIG. 5, the angle of the straight line Da with respect to the Y direction is indicated by φ₁, and the angle of the straight line Da with respect to the straight line L₁ is indicated by φ₂. The angle φ₁ corresponds to the rotation angle of the contact plug CW₃₁. As described above, each contact plug CW of the first embodiment is arranged so that the major axis of the ellipse is tilted with respect to the Y direction. Therefore, the angle φ₁ satisfies the relation of 0°<φ₁<90°, similarly to the angle θ.

However, in the first embodiment, the major axis of the contact plug CW₃₁ and the straight ling L₁ are tilted in opposite directions with respect to the Y direction. Specifically, while the straight line L₁ is rotated clockwise by the angle θ with respect to the Y direction, the straight line Da is rotated counterclockwise by the angle φ₁ with respect to the Y direction. As a result, the relation of φ₂=φ₁+θ is established among θ, φ₁ and φ₂.

In the first embodiment, the relation among them is assumed to be established for the straight lines L₂ to L₄ other than the straight line L₁ and each contact plug CW other than CW₃₁. For example, the straight lines L₂, L₃ and L₄ are all rotated clockwise by the angle θ with respect to the Y direction, and the contact plugs CW₃₃, CW₃₂ and CW₃₄ are rotated counterclockwise by the angle φ₁ with respect to the Y direction (i.e. the direction in which the device regions 111 extend). This is specifically shown in FIG. 4.

Although the tilts of all the straight lines L₁ to L₄ with respect to the Y direction are set to the angle θ in the first embodiment, they may be set to angles different from each other. In addition, although the rotation angles of all the contact plugs CW within the contact region R_(D) are set to φ₁, they may be set to angles different from each other. However, to improve the lithography margin, the former tilts desirably have the same angle, and the latter tilts also desirably have the same angle.

In addition, although the contact plugs CW and the straight lines L₁ to L₄ are tilted in opposite directions with respect to the Y direction in the first embodiment, they may be tilted in the same direction. However, from the viewpoint of easily preventing a short of the contact plugs CW in the same partial region, the contact plugs CW and the straight lines L₁ to L₄ are desirably tilted in opposite directions.

Further, in the first embodiment, all the contact plugs CW in the contact region R_(D) are rotated counterclockwise with respect to the Y direction, so that the contact plugs CW in the first partial regions R₁ and the contact plugs CW in the second partial regions R₂ are tilted in the same direction. According to the calculation by the present inventors, it is found that such arrangement can realize a more preferable contact arrangement than an arrangement in which the contact plugs CW in the first partial regions R₁ and the contact plugs CW in the second partial regions R₂ are tilted in opposite directions. The detail will be described later.

Likewise, the contact plugs CW in the irregular five-contact oblique arrangements adjacent to each other are tilted in the same direction in the first embodiment. For example, the contact plugs CW₂₁, CW₂₂, . . . , CW₂₅ in the irregular five-contact oblique arrangement and the contact plugs CW₃₁, CW₃₂, . . . , CW₃₅ in the irregular five-contact oblique arrangement are tilted in the same direction. According to the calculation by the present inventors, it is found that such arrangement can realize a more preferable contact arrangement than an arrangement in which the contact plugs CW in these irregular five-contact oblique arrangements are tilted in opposite directions. The detail will be described later.

(3) Method of Determining Rotation Angle of Contact Plugs CW

Next, a method of determining the rotation angle of contact plugs CW will be described with reference to FIGS. 6 and 7.

FIGS. 6 and 7 are a plan view and a graph for explaining the method of determining the rotation angle of the contact plugs CW, respectively. Hereinafter, the method will be described by taking the contact plugs CW₃₁, CW₃₂, CW₂₃ and CW₂₄ as an example.

In FIG. 6, the symbol A indicates a distance between CW₃₁ and CW₂₃ on a straight line connecting the center points G of CW₃₁ and CW₂₃. Further, the symbol C indicates a distance between CW₃₁ and CW₂₄ on a straight line connecting the center points G of CW₃₁ and CW₂₄. In addition, the contact plugs before rotation are shown by dotted lines.

Each of the distances A and C corresponds to a distance between the contact plugs CW disposed on the device regions 111 which are adjacent to each other in the X direction. However, the distance A corresponds to a distance between the contact plugs CW in the same partial region, while the distance C corresponds to a distance between the contact plugs CW in different partial regions. The distances between the contact plugs CW other than CW₃₁, CW₂₃ and CW₂₄ are also equal to the distances A and C.

In addition, FIG. 6 shows a first tangent line contacted with the contact plug CW₂₄ before rotation and extending in the Y direction, and a second tangent line contacted with the contact plug CW₂₄ after rotation and extending in the Y direction. The distance between these tangent lines is indicated by the symbol B. The distance between such tangent lines of each contact plug CW other than CW₂₄ is also equal to the distance B.

Further, the symbol δ in FIG. 6 indicates the angle between the straight line connecting the center points G of CW₃₁ and CW₂₄ and the straight line Y₁. Although the rotation angle φ₁ of each contact plug CW in FIG. 6 is set to a value larger than the angle δ, it may be set to a value smaller than the angle δ.

On the other hand, FIG. 7 shows the distances A, B and C which are changed according to the rotation angle φ₁ of the contact plugs CW. In FIG. 7, the relation between the rotation angle φ₁ and the distances A, B and C is plotted. The relation is obtained by the calculation by the present inventors.

As described above, the distance B corresponds to a distance between the tangent lines before and after the rotation of the contact plugs CW (see FIG. 6). Therefore, the distance B corresponds to a change amount of the length how long a portion of each contact plug CW is extended off a device region 111 in the X direction with the rotation. The length of the extended-off portion is desirably set to a small value, from the viewpoint of securing the contact area between each contact plug CW and a device region 111 and of preventing a short between each contact plug CW and the adjacent device region 111. Therefore, the distance B is desirably set to a small value.

Referring to FIG. 7, the distance B increases with the rotation angle φ₁. Therefore, when determining the rotation angle φ₁, the angle φ₁ is desirably set to a small value, for example, set to a value within the range of 0° to 30°.

In addition, referring to FIG. 7, when the rotation angle 4 has a relatively small value (e.g., 0° to 30°), the distance A has a shorter value than the distance C. Therefore, when the rotation angle φ₁ has a relatively small value, the angle φ₁ is desirably set so that the distance A is increased with the rotation, in order to prevent a short of the contact plugs CW.

Referring to FIG. 7, the distance A increases with the rotation angle φ₁. Therefore, from the viewpoint of increasing the distance A, the rotation angle φ₁ is desirably set to a large value. However, from the viewpoint of suppressing the increase of the distance B, the rotation angle φ₁ is desirably set to a small value as described above.

Further, referring to FIG. 7, the distance C is hardly changed while the rotation angle φ₁ is small. On the other hand, when the rotation angle φ₁ is larger than a certain angle, the distance C tends to be increased. It should be noted that the distance C is larger than the distance A, so that a short of the contact plugs CW tends to depend mainly on the distance A.

Therefore, in the first embodiment, the rotation angle φ₁ is determined in consideration of the balance between the increase of the distance A and the suppression of the increase of the distance B. For example, the rotation angle φ₁ is set to 5° to 15°, more specifically, set to 10°.

FIG. 7 shows the calculated result of the distance A where the contact plugs CW₂₁, CW₂₂, . . . , CW₂₅ in the five-contact oblique arrangement and the contact plugs CW₃₁, CW₃₂, . . . , CW₃₅ in the five-contact oblique arrangement are tilted in the same direction, and the calculated result of the distance A where those contact plugs are tilted in opposite directions. In the latter case, the contact plug CW₃₁ is rotated counterclockwise with respect to the Y direction, and the contact plug CW₂₃ is rotated clockwise with respect to the Y direction, for example. In FIG. 7, the former case is indicated by white circle plots, and the latter case is indicated by black circle plots.

As understood from those plots, the white circle plots have a higher increase rate of the distance A with rotation than the black circle plots. Therefore, from the viewpoint of increasing the distance A with rotation, the contact plugs CW in the five-contact oblique arrangements adjacent to each other are desirably tilted in the same direction (i.e., rotated in the same direction with respect to the Y direction).

From the same viewpoint, the contact plugs CW in the first partial regions R₁ and the contact plugs CW in the second partial regions R₂ are desirably tilted in the same direction to increase the distance A.

(4) Method of Changing Ellipse Dimensions of Contact Plugs

In the above description with reference to FIGS. 6 and 7, the contact plugs CW are assumed to be rotated without changing the ellipse dimensions. Hereinafter, referring to FIGS. 8A to 8C, the case where the ellipse dimensions of the contact plugs CW are changed with the rotation of the contact plugs CW will be described.

FIGS. 8A to 8C are plan views for explaining a method of changing the ellipse dimensions of each contact plug CW.

FIG. 8A shows the contact plug CW before the rotation. FIG. 8A shows a major radius “a” and a minor radius “b” as the ellipse dimensions of the contact plug CW.

Then, FIG. 8B shows a state where the contact plug CW of FIG. 8A is rotated without changing the ellipse dimensions “a” and “b”. Further, FIG. 8B shows the distance B between the tangent lines before and after the rotation of the contact plug CW.

Then, FIG. 8C shows a state where the ellipse dimensions “a” and “b” of the contact plug CW of FIG. 8B are changed. In FIG. 8C, the major radius “a” is changed to a longer major radius “a′”, and the minor radius “b” is changed to a shorter minor radius “b′”. If the rotation angle φ₁ is increased so that the major radius “a” is extended off the device region 111, the major radius “a” is changed to a shorter major radius, and the minor diameter “b” is changed to a longer minor radius.

In the first embodiment, the ellipse dimensions “a′ and b′” are determined under the conditions in which the distance B is returned to 0 by the dimension change, and the ellipse areas before and after the dimension change have the same value. The former condition has an effect of preventing a short of the contact plug CW and the adjacent device region 111. In addition, the latter condition has an effect of reducing the change of the contact area between the contact plug CW and the device region 111 with the rotation.

Further, to improve the lithography margin, each of the ellipse dimensions “a′ and b′” is desirably set to the same dimension for all the contact plugs CW in the contact region R_(D), similarly to the rotation angle φ₁. When the dimension change is not performed, each of the ellipse dimensions “a and b” is desirably set to the same dimension for all the contact plugs CW in the contact region R_(D).

(5) Description of Symbols and Parameters in FIG. 4

Next, symbols and parameters shown in FIG. 4 will be described.

In FIG. 4, the width of each contact plug CW in the Y direction is indicated by W_(Y), and the pitch between the contact plugs CW in the Y direction is indicated by P_(Y). If the rotation angle of the contact plug CW is 0°, the width W_(Y) represents a diameter of the contact plug CW along the major axis. Also, P_(Y) specifically represents the pitch in the Y direction between the contact plugs CW arranged in the same partial region. In the first embodiment, the contact plugs CW in each partial region is arranged so that the pitch P_(Y) is a constant value, and is longer than the width W_(Y) (P_(Y)>W_(Y)). Further, the first partial regions R₁ and the second partial regions R₂ have the same pitch P_(Y) and the same width W_(Y), so that the lithography margin can be improved. In the first embodiment, the partial regions in the contact region R_(D) have the same pitch P_(Y) and the same width W_(Y).

Further, in FIG. 4, the distance between the contact plugs CW in the Y direction is indicated by the symbol n. More specifically, n represents the distance in the Y direction between the contact plugs CW arranged in the same partial region and adjacent to each other in the X direction. The relation of P_(Y)=W_(Y)+n is established between the width W_(Y), the pitch P_(Y), and the distance n. In the first embodiment, the distance n is set to be the value in which electric breakdown voltage between the contact plugs CW can be sufficiently secured. In addition, in FIG. 4, the distance in the X direction between the contact plugs CW arranged in the same partial region and adjacent to each other in the X direction is indicated by a symbol m. In the contact region R_(D) of FIG. 4, the shortest distance between the contact plugs CW which are adjacent to each other in the X direction is represented by (n²+m²)^(1/2).

FIG. 4 also shows a center point (center axis) G of each contact plug CW. The center axis G is located at the position passing through the intersection of the major axis and the minor axis of each contact plug CW.

FIG. 4 also shows straight lines X, to X₅ extending in the X direction. As shown in FIG. 4, the center points G of the contact plugs CW in the first partial regions R₁ are located on the straight lines X₁, X₃, and X₅, and the center points G of the contact plugs CW in the second partial regions R₂ are located on the straight lines X₂ and X₄. In this way, each of the straight lines X₁ to X₅ (further, the straight lines L₁ to L₄) passes through the center points G of contact plugs CW. Hereinafter, the contact plugs CW arranged on the straight lines X₁ to X₅ will be called the first-stage to the fifth-stage contact plugs CW. Desirably, the intervals in the Y direction between the straight lines X₁ to X₅ are all equal to each other. This is because the lithography margin is improved by arranging the contact plugs CW regularly.

In addition, in the contact region R_(D) of FIG. 4, three-contact oblique arrangements and two-contact oblique arrangements are alternately repeated, thereby realizing a structure in which five contact plugs CW are obliquely arranged on five device regions 111 at one device region interval, so as to be arranged on the straight lines X₁ to X₅ in this order, respectively. For example, the contact plugs CW₂₁, CW₂₂, CW₂₃, CW₂₄, and CW₂₅ are obliquely arranged on five device regions 111 at one device region interval. In addition, the contact plugs CW₃₁, CW₃₂, CW₃₃, CW₃₄, and CW₃₅ are obliquely arranged on five device regions 111 at one device region interval.

Further, in the first embodiment, the straight line X₂ passes between the first-stage contact plugs CW and the third-stage contact plugs CW in the first partial regions R₁, and the second-stage contact plugs CW in the second partial regions R₂ are arranged on this straight line X₂. Similarly, the straight line X₄ passes between the third-stage contact plugs CW and the fifth-stage contact plugs CW in the first partial regions R₁, and the fourth-stage contact plugs CW in the second partial regions R₂ are arranged on this straight line X₄.

On the other hand, the straight line X₃ passes between the second-stage contact plugs CW and the fourth-stage contact plugs CW in the second partial regions R₂, and the third-stage contact plugs CW in the first partial regions R₁ are arranged on this straight line X₃.

In this way, in the first embodiment, the M-th-stage contact plugs CW are arranged on a straight line which is parallel to the X direction and passes between the (M−1)-th-stage contact plugs CW and the (M+1)-th-stage contact plugs CW, where M denotes an integer of 2 to 4. As described above, such arrangement can be realized by making the pitch P_(Y) longer than the width W_(Y). In addition, the even-stage contact plugs CW are arranged between the odd-stage contact plugs CW, so that the interval between the contact plugs CW which are adjacent in the X direction can be increased.

Further, in FIG. 4, the width of each contact plug CW in the X direction is indicated by W_(X), and the pitch between the contact plugs CW in the X direction is indicated by P_(X). If the rotation angle of the contact plug CW is 0°, the width W_(X) represents a diameter of the contact plug CW along the minor axis. Also, P_(X) specifically represents the pitch in the X direction between the contact plugs CW which are arranged on the same straight line extending in the X direction and are adjacent to each other in the X direction. In other words, P_(X) represents the pitch between the contact plugs CW in the X direction when a first partial region R₁ and a second partial region R₂ are integrated. In the first embodiment, the pitch P_(X) has a length equal to the total width of five device regions 111 and five isolation insulators 112. In other words, four device regions 111 are sandwiched between the contact plugs CW which are arranged on the same straight line extending in the X direction and are adjacent to each other in the X direction.

Further, in the irregular (N₁+N₂)-contact staggered arrangement, the pitch P_(X) has a length equal to the total width of (N₁+N₂) device regions 111 and (N₁+N₂) isolation insulators 112. In other words, (N₁+N₂−1) device regions 111 are sandwiched between the contact plugs CW which are arranged on the same straight line extending in the X direction and are adjacent to each other in the X direction.

In FIG. 4, the distance in the Y direction between the upper ends of the first-stage contact plugs CW and the lower ends of the fifth-stage contact plugs CW is indicated by α. Also, the distance in the Y direction between the select gates SG_(D) and SG_(D)′ is indicated by β. The relation of α<β is established between the distances α and β. In addition, the relation of α=3 W_(Y)+2 n is established between the distance α, the width W_(Y), and the distance n.

(6) Comparison with First to Third Comparative Examples

Next, the semiconductor memory device of the first embodiment will be compared with semiconductor memory devices of first to third comparative examples.

FIG. 9 is a plan view showing the structure of the semiconductor memory device of the first comparative example.

The contact region R_(D) of FIG. 9 has a contact arrangement in which three-contact oblique arrangements are periodically repeated in the X direction (called simple three-contact staggered arrangement).

In the contact arrangement of the first comparative example, the density of the contact plugs CW is greatly different between an end portion such as the first-stage or the third-stage and a center portion such as the second-stage. Due to the presence of such density difference, variations in lithography and processing are easily caused at the time of forming the contact plugs CW.

FIG. 10 is a plan view showing the structure of the semiconductor memory device of the second comparative example.

The contact region R_(D) of FIG. 10 has a contact arrangement in which five-contact oblique arrangements are periodically repeated in the X direction (called simple five-contact staggered arrangement).

The contact arrangement of the second comparative example has the distance β between the select gates longer than that in the first comparative example.

On the contrary, in the third comparative example and the first embodiment, the contact arrangement shown in FIGS. 4 and 11 (irregular five-contact staggered arrangement) is adopted. FIG. 11 is a plan view showing the structure of the semiconductor memory device of the third comparative example.

In the third comparative example and the first embodiment, five contact plugs CW forming one oblique arrangement are not arranged on five successive device regions 111, but arranged on five contact plugs CW at one device region interval. In addition, the distance n of the third comparative example and the first embodiment can be set to the same value as the distance n of the first comparative example, so that electric breakdown voltage between the contact plugs CW cannot be lowered. This is because in the first and third comparative examples and the first embodiment, the shortest distance between the contact plugs CW which are adjacent to each other in the X direction is (n²+m²)^(1/2). Consequently, in the third comparative example and the first embodiment, the distance a can be set to a distance shorter than that of the second comparative example (simple five-contact staggered arrangement), and equal to that of the first comparative example (simple three-contact staggered arrangement) (see FIGS. 4 and 11). In other words, in the third comparative example and the first embodiment, the relation of α=3 W_(Y)+2 n is established among the distance α, the width W_(Y), and the distance n. Therefore, according to the third comparative example and the first embodiment, the distance n and the pitch P_(Y) can be sufficiently secured without increasing the distance β between the select gates.

The above effect can secure the distance n and the pitch P_(Y) in the first embodiment in which the contact plugs CW are rotated, more sufficiently than the third comparative example in which the contact plugs CW are not rotated. As understood from FIGS. 4 and 11, the value of the width W_(Y) of the first embodiment is smaller than that of the third comparative example. In addition, although the distance n of the first embodiment is increased with the rotation of the contact plugs CW, the distance n can be returned to the value before the rotation by reducing the distance between the contact plugs CW in the Y direction after the rotation. Therefore, the distance n of the first embodiment can be set to the same value as the distance n of the third comparative example. As a result, according to the first embodiment, the distances a and β can be shorter than those of the third comparative example.

As described above, according to the first embodiment, the distance n and the pitch P_(Y) are sufficiently secured, so that the short of the contact plugs CW and the decrease in electric breakdown voltage between the contact plugs CW can be suppressed. Further, according to the first embodiment, the distance β between the select gates is shortened, so that the increase of the chip area can be suppressed. Further, according to the first embodiment, the contact plugs CW are rotated with respect to the Y direction, so that the distance β between the select gates can be set shorter than that where the contact plugs CW are not rotated.

Also, in the first embodiment, the contact arrangement in which the three-contact oblique arrangements and the two-contact oblique arrangements are alternately repeated is adopted. Consequently, in the first embodiment, a close structure is realized in which a contact plug CW in a second partial region R₂ is arranged in each lattice cell of a lattice, which is formed by 1) a straight line passing the contact plugs CW that are arranged in a first partial region R₁ and are arranged on the same straight line, 2) a straight line passing the contact plugs CW that are arranged in its neighboring first partial region R₁ and are arranged on the same straight line, and 3) and straight lines passing the contact plugs CW that are arranged in these first partial regions R₁ and are adjacent to each other in the X direction. In addition, a close structure is realized in which a contact plug CW in a first partial region R₁ is arranged in each lattice cell of a lattice, which is formed by 1) a straight line passing the contact plugs CW that are arranged in a second partial region R₂ and are arranged on the same straight line, 2) a straight line passing the contact plugs CW that are arranged in its neighboring second partial region R₂ and are arranged on the same straight line, and 3) and straight lines passing the contact plugs CW that are arranged in these second partial regions R₂ and are adjacent to each other in the X direction. For example, in FIG. 4, the contact plug CW₃₂ is arranged in a lattice cell S₁, and the contact plug CW₂₄ is arranged in a lattice cell S₂. Such close structures have an advantage that the density difference between the contact plugs CW near the select gates SG (end portion) and the contact plugs CW near the center between the select gates SG (center portion) can be reduced.

Therefore, according to the first embodiment, variations in lithography and processing can be reduced at the time of forming the contact plugs CW, so that the lithography accuracy and the processing accuracy can be improved.

As described above, in each partial region of the first embodiment, N contact plugs CW are arranged on N successive device regions 111 so as to be arranged on the straight line being non-parallel to the X and Y directions, where N denotes an integer of 2 or more. Further, the contact region R_(D) of the first embodiment includes the partial regions of two types whose values of N are different. More specifically, the value of N is set to N₁ or N₂ for each partial region, so that the irregular (N₁+N₂)-contact staggered arrangement can be realized. However, in the first embodiment, contact arrangements other than such irregular (N₁+N₂)-contact staggered arrangement may be adopted. Also, the contact region R_(D) of the first embodiment may include the partial regions of three or more types whose values of N are different.

(7) Structure of Contact Region R_(D) (Lattice Arrangement)

Hereinafter, referring to FIG. 4, the structure of the contact region R_(D) will be described in more detail.

S₁ and S₂ shown in FIG. 4 represent lattice cells of a lattice that is formed by the straight lines L₁ and L₂ extending in the same direction which are non-parallel to the X and Y directions, and by the straight lines X₁, X₃, and X₅ extending in the X direction. The contact plugs CW in the first partial regions R₁ are arranged at the respective lattice points of the lattice. For example, the contact plugs CW₃₁, CW₂₃, CW₃₃, and CW₄₁ are arranged at the respective lattice points forming the lattice cell S₁, and the contact plugs CW₂₃, CW₁₅, CW₂₅, and CW₃₃ are arranged at the respective lattice points forming the lattice cell S₂.

In this way, in the first embodiment, the contact plugs CW in the first partial regions R₁ are arranged at the respective lattice points of the lattice that is formed by first straight lines extending in the same direction which are non-parallel to the X and Y directions, and by second straight lines extending in the X direction. Here, the first straight lines include the straight lines L₁ and L₂ and other straight lines which are parallel to the straight lines L₁ and L₂. Also, the second straight lines include the straight lines X₁, X₃, and X₅. In the first embodiment, with such contact arrangement, regular arrangements of the contact plugs CW in the first partial regions R₁ can be realized. Specifically, the contact plugs CW in the first partial regions R₁ are arranged so as to form a plane rhombic lattice (parallelogram lattice).

Hereinafter, the direction in which the straight lines L₁ and L₂ extend will be referred to as a third direction with respect to the Y direction (the first direction) and the X direction (the second direction).

In addition, in FIG. 4, a contact plug CW in a second partial region R₂ is arranged in each lattice cell of the above lattice. For example, the contact plug CW₃₂ is arranged in the lattice cell S₁, and the contact plug CW₂₄ is arranged in the lattice cell S₂.

In this way, in the first embodiment, a contact plug CW in a second partial region R₂ is arranged in each lattice cell of the lattice which is formed by the first and second straight lines. Consequently, in the first embodiment, regular arrangements of the contact plugs CW in the second partial regions R₂ can also be realized. Specifically, the contact plugs CW in the second partial regions R₂ are arranged so as to form a face-centered rhombic lattice together with the contact plugs CW in the first partial regions R₁. Further, two or more contact plugs CW in a second partial region R₂ may be arranged in each lattice cell of the lattice.

In addition, S₃ shown in FIG. 4 represents lattice cell of a lattice that is formed by the straight lines L₃ and L₄ extending in the third direction and by the straight lines X₂ and X₄ extending in the X direction. The contact plugs CW in the second partial regions R₂ are arranged at the respective lattice points of this lattice. For example, the contact plugs CW₃₂, CW₂₄, CW₃₄, and CW₄₂ are arranged at the respective lattice points forming the lattice cell S₃.

In this way, in the first embodiment, the contact plugs CW in the second partial regions R₂ are arranged at the respective lattice points of the lattice that is formed by third straight lines which extend in the third direction and pass between the first straight lines, and by fourth straight lines which extend in the X direction and pass between the second straight lines. Here, the third straight lines include the straight lines L₃ and L₄, and other straight lines which are parallel to the straight lines L₃ and L₄. Also, the fourth straight lines include the straight lines X₂ and X₄. In the first embodiment, with such contact arrangement, regular arrangements of the contact plugs CW in the second partial regions R₂ can be realized. Specifically, the contact plugs CW in the second partial regions R₂ are arranged so as to form a plane rhombic lattice.

Further, in the first embodiment, the first and the third straight lines both extend in the third direction and are parallel to each other. However, these straight lines may not be parallel to each other. For example, the first straight lines may extend in the third direction, and the third straight lines may extend in the fourth direction which is non-parallel to the first to the third directions.

(8) Effects of First Embodiment

Finally, effects of the first embodiment will be described.

As described above, in each partial region of the first embodiment, N contact plugs CW are arranged on N successive device regions 111 so as to be arranged on the straight line being non-parallel to the X and Y directions. Further, the contact region R_(D) of the first embodiment includes the partial regions of at least two types whose values of N are different. More specifically, the value of N is set to be N₁ or N₂ for each partial region, so that the irregular (N₁+N₂)-contact staggered arrangement can be realized.

According to the first embodiment, the irregular (N₁+N₂)-contact staggered arrangement is adopted, so that the distance α can be shorter than in the case where the simple (N₁+N₂)-contact staggered arrangement is adopted (for example, see FIGS. 4 and 10). The distance β between the select gates largely depends on the distance α. Therefore, according to the first embodiment, while the distance β between the select gates is shortened, the distances n and m and the pitch P_(Y) can be sufficiently secured. Further, according to the first embodiment, the major axis of each contact plug CW is tilted with respect to the Y direction, so that the distances α and β can be set shorter than those where the major axis is parallel to the Y direction.

By sufficiently securing the distances n and m and the pitch P_(Y), the short of the contact plugs CW and the decrease in breakdown voltage between the contact plugs CW can be suppressed. In addition, by decreasing the distance β between the select gates, the increase of the chip area can be suppressed.

In addition, according to the first embodiment, the contact plugs CW are rotated with respect to the direction in which the device regions 111 extend (Y direction), so that the distance between the respective contact plugs CW can be increased. In particular, according to the first embodiment, the respective contact plugs CW and the straight lines L₁ to L₄ are rotated in opposite directions with respect to the Y direction, so that the distance between the contact plugs CW can be further increased.

Therefore, according to the first embodiment, the short of the contact plugs CW and the decrease in breakdown voltage between the contact plugs CW can be suppressed. Further, according to the first embodiment, the irregular five-contact staggered arrangement is adopted, so that the increase of the chip area can be suppressed.

In addition, according to the first embodiment, variations in lithography and processing can be reduced at the time of forming the contact plugs CW, so that the lithography accuracy and the processing accuracy can be improved.

In the first embodiment, each contact plug CW has the planar shape of an ellipse. However, each contact plug CW may have a planar shape of a rectangle. In this case, in the above description, the major axis can be replaced with a long side of the rectangle, and the minor axis can be replaced with a short side of the rectangle. In this case, each contact plug CW having the planar shape of the rectangle is arranged so that the long side is tilted with respect to the Y direction.

In addition, the ellipse shape of each contact plug CW is not limited to a mathematically strict ellipse, and is assumed to include any ellipse shape obtained by rounding four corners of a rectangle. Examples of such shape include a shape obtained by coupling one rectangle and two semicircles, and an egg shape (oval shape).

Hereinafter, a second embodiment which is a modification of the first embodiment will be described focusing on differences between the first and second embodiments.

Second Embodiment

FIG. 12 is a plan view showing a structure of a semiconductor memory device of the second embodiment.

In each first partial region R₁ of FIG. 4, three contact plugs CW are arranged in the oblique arrangement. Also, in each second partial region R₂, two contact plugs CW are arranged in the oblique arrangement.

On the contrary, in each first partial region R₁ of FIG. 12, four contact plugs CW are arranged in an oblique arrangement. Also, in each second partial region R₂ of FIG. 12, three contact plugs CW are arranged in an oblique arrangement. The rotation direction of each contact plug GW with respect to the Y direction is same as that in the first embodiment.

Consequently, in FIG. 12, a contact arrangement in which four-contact oblique arrangements and three-contact oblique arrangements are alternately repeated (called irregular seven-contact staggered arrangement) can be realized. In FIG. 12, the pitch P_(X) has a length equal to the total width of seven device regions 111 and seven isolation insulators 112.

In the second embodiment, seven contact plugs CW in a seven-contact staggered arrangement are not arranged on seven successive device regions 111, but arranged on seven device regions 111 at two device region interval. Consequently, in the second embodiment, the distance a can be set to be a distance shorter than that of the simple seven-contact staggered arrangement, and equal to that of the simple four-contact staggered arrangement. Therefore, according to the second embodiment, similarly to the first embodiment, while the distance β between the select gates is shortened, the distance n and the pitch P_(Y) can be sufficiently secured.

According to the second embodiment, the distance n and the pitch P_(Y) can be sufficiently secured, so that the short of the contact plugs CW and the decrease in breakdown voltage between the contact plugs CW can be suppressed. In addition, according to the second embodiment, the distance β between the select gates can be shortened, so that the increase of the chip area can be suppressed.

FIG. 12 shows lattice cells S₁, S₂, and S₃ of a lattice that is formed by the straight lines L₁ and L₂ extending in the third direction and by the straight lines X₁, X₃, X₅, and X₇ extending in the X direction. Similarly to the first embodiment, the contact plugs CW in the first partial regions R₁ are arranged at the respective lattice points of this lattice. Consequently, the regular arrangement of the contact plugs CW in the first partial regions R₁ can be realized.

Further, FIG. 12 shows lattice cells S₄ and S₅ of a lattice that is formed by the straight lines L₃ and L₄ extending in the third direction, and by the straight lines X₂, X₄, and X₆ extending in the X direction. Similarly to the first embodiment, the contact plugs CW in the second partial regions R₂ are arranged at the respective lattice points of this lattice. Consequently, similarly to the contact plugs CW in the first partial region R₁, the regular arrangement of the contact plugs CW in the second partial regions R₂ can be realized.

Effects of Second Embodiment

Finally, effects of the second embodiment will be described.

As described above, in each partial region of the second embodiment, the irregular (N₁+N₂)-contact staggered arrangement (here N₁+N₂=7) is adopted, so that the distance a can be shorter than in a case where the simple (N₁+N₂)-contact plug staggered arrangement is adopted. Therefore, according to the second embodiment, while the distance β between the select gates is shortened, the distances n and m and the pitch P_(Y) can be sufficiently secured. Further, according to the second embodiment, the major axis of each contact plug CW is tilted with respect to the Y direction, so that the distances α and β can be set shorter than those where the major axis is parallel to the Y direction. They are similar to the effects in the first embodiment.

By sufficiently securing the distances n and m and the pitch P_(Y), the short of the contact plugs CW and the decrease in breakdown voltage between the contact plugs CW can be suppressed. In addition, by decreasing the distance β between the select gates, the increase of the chip area can be suppressed.

Therefore, according to the second embodiment, the short of the contact plugs CW and the decrease in breakdown voltage between the contact plugs CW can be suppressed. Further, according to the second embodiment, the irregular (N₁+N₂)-contact staggered arrangement is adopted, so that the increase of the chip area can be suppressed.

In addition, according to the second embodiment, variations in lithography and processing can be reduced at the time of forming the contact plugs CW, so that the lithography accuracy and the processing accuracy can be improved. In particular, when the value of N₁+N₂ is increased, the density difference between the contact plug CW near the select gates SG (end portion) and the contact plug CW near the center between the select gates SG (center portion) tends to be increased. For this reason, as the value of N₁+N₂ is increased, the effect of improving the lithography accuracy and the processing accuracy becomes greater.

In addition, according to the second embodiment, the contact plugs CW are rotated with respect to the direction in which the device regions 111 extend (Y direction), so that the distance between the respective contact plugs CW can be increased. In particular, according to the second embodiment, the respective contact plugs CW and the straight lines L₁ to L₄ are rotated in opposite directions with respect to the Y direction, so that the distance between the contact plugs CW can be further increased.

While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel devices described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the devices described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions. 

1. A semiconductor memory device comprising: a substrate; device regions formed in the substrate to extend in a first direction which is parallel to a principal plane of the substrate; select gates disposed on the substrate to extend in a second direction which is perpendicular to the first direction; and a contact region provided between the select gates on the substrate and including contact plugs disposed on the respective device regions, wherein the contact region includes partial regions, in each of which N contact plugs are disposed on N successive device regions to be arranged on a straight line being non-parallel to the first and second directions, where N is an integer of 2 or more, the contact region includes the partial regions of at least two types whose values of N are different, and each of the contact plugs has a planar shape of an ellipse or rectangle, and is arranged so that a major axis of the ellipse or a long side of the rectangle is tilted with respect to the first direction.
 2. The device of claim 1, wherein the major axis or the long side of each contact plug and the straight line are tilted in opposite directions with respect to the first direction.
 3. The device of claim 1, wherein a tilt angle of the major axis or the long side of each contact plug with respect to the first direction is larger than 0° and is smaller than 30°.
 4. The device of claim 1, wherein the contact plugs included in the same partial region are tilted in the same direction with respect to the first direction.
 5. The device of claim 1, wherein the contact plugs in each partial region are disposed so that a pitch between the contact plugs in the first direction is a constant value.
 6. The device of claim 1, wherein the straight lines of the partial regions are parallel to each other.
 7. The device of claim 1, wherein the contact region includes: first partial regions, in each of which N₁ contact plugs are disposed on N₁ successive device regions to be arranged on a straight line which is non-parallel to the first and second directions, where N₁ is a constant integer of 2 or more; and second partial regions, in each of which N₂ contact plugs are disposed on N₂ successive device regions to be arranged on a straight line which is non-parallel to the first and second directions, where N₂ is a constant integer of 2 or more and is different from N₁, wherein the first and second partial regions are alternately disposed in the contact region along the second direction.
 8. The device of claim 7, wherein each contact plug in the first partial regions and each contact plug in the second partial regions are tilted in the same direction with respect to the first direction.
 9. The device of claim 7, wherein a difference between N₁ and N₂ is +1 or −1.
 10. The device of claim 7, wherein the contact plugs are disposed so that N₁+N₂−1 device regions are sandwiched between the contact plugs adjacent in the second direction.
 11. The device of claim 7, wherein the contact plugs in each of the first and second partial regions are disposed so that a pitch between the contact plugs in the first direction is a constant value.
 12. The device of claim 7, wherein the straight lines of the first and second partial regions are parallel to each other.
 13. The device of claim 7, wherein each contact plug in the second partial regions is arranged on a straight line which is parallel to the second direction and passes between the contact plugs in the first partial regions.
 14. The device of claim 7, wherein the contact plugs in each first partial region are arranged at respective lattice points of a lattice which is formed by: first straight lines extending in a third direction which is non-parallel to the first and second directions; and second straight lines extending in the second direction.
 15. The device of claim 14, wherein the contact plugs in each second partial region are arranged at respective lattice points of a lattice which is formed by: third straight lines extending in the third direction and passing between the first straight lines; and fourth straight lines extending in the second direction and passing between the second straight lines.
 16. The device of claim 15, wherein one or more contact plugs in the second regions are disposed in each lattice cell of the lattice formed by the first and second straight lines.
 17. The device of claim 15, wherein a total number of the second and fourth straight lines of the contact region is N₁+N₂.
 18. The device of claim 14, wherein the contact plugs in each second partial region are arranged at respective lattice points of a lattice which is formed by: third straight lines extending in a fourth direction which is non-parallel to the first to third directions and passing between the first straight lines; and fourth straight lines extending in the second direction and passing between the second straight lines.
 19. The device of claim 18, wherein one or more contact plugs in the second regions are disposed in each lattice cell of the lattice formed by the first and second straight lines.
 20. The device of claim 18, wherein a total number of the second and fourth straight lines of the contact region is N₁+N₂. 